There are many applications in which it is desired to use a single laser beam of multiple wavelengths in the visible range of the electromagnetic spectrum. As a single beam, all wavelengths are co-propagating, i.e. propagate in the same direction and along a common propagation axis. Examples of such applications are the production of full-color images (motion picture video displays, printing and repro-graphics, etc.) and various types of fluorescence-based analyses in the field of biotechnology, such as flow-cytometry and bio scanners, and fluorescence microscopy.
Argon-ion lasers (Ar-ion lasers) have traditionally been used for many of the above applications, since they can produce multiple wavelengths in the blue and green (457, 488 and 514 nm) propagating in the same direction along a common propagation axis. Further, Ar-ion lasers provide good beam quality and high stability.
However, Ar-ion lasers are of large size, have a limited lifetime (typically <5000 hrs.), have a high power consumption (typically >1 kW of electrical power for <100 mW optical output power), and require frequent maintenance.
Recently, it has been proposed to use solid-state lasers as an alternative to Ar-ion lasers in an attempt to overcome the above-mentioned drawbacks. Among the solid-state lasers mentioned in this context are optically or electrically pumped semiconductor lasers (e.g. VCSELs or standard single-mode edge emitters) combined with elements for frequency conversion, and up-conversion fiber lasers, which are all capable of producing a laser beam in the blue or green region of the spectrum with characteristics similar to those of the Ar-ion laser.
Another proposed alternative to Ar-ion lasers is sum-frequency mixing of two diode-pumped Nd-doped laser crystals (see for example WO 02/103863).
Diode pumped laser crystals provide excellent beam properties and long operational life-time, as well as good power efficiency. However, a general problem of these solid-state lasers is that they normally only emit one wavelength. For applications where it is required to have more than one wavelength in a single beam, it then becomes necessary to combine multiple laser units and merge the output beams from these units into a single beam. Such combination could be made, for example, by means of rather complicated mirror set-ups or by coupling the radiation into an optical fiber.
In addition, a specific problem of sum-frequency mixing according to the above-referenced WO 02/103863, is that two separate pump diodes are required, thereby adding to the complexity and cost of the laser arrangement.